Three dimensional shapes of hydrogen-air flames within millimetric Hele Shaw cells.

Premixed hydrogen-air laminar flames with equivalence ratios ranging from 0.32 to 2.68 adopt different shapes during propagation in a gap (from 1. 5 mm to 4. 2 mm) between two plates. Thermodiffusive and Darrieus–Landau instabilities are visible for lean mixture while Darrieus–Landau is visible at higher equivalent ratio. Using for the first time simultaneous schlieren visualisation in this configuration, we were able to make two important observations: (i) either symmetric or asymmetric shapes are observed and quantified within the gap as a function of equivalence ratio and gap thickness; (ii) 3D flame surface for unstable laminar flame was estimated. These results validate (or invalidate) the quasi-2D assumptions made in Hele-Shaw cells. Linear stability analysis allowed us to find an a priori criteria differentiating symmetric and asymmetric cases. Finally, curvature effect on flame speed enhancement relative to surface amplification is estimated for unstable hydrogen-air laminar flames. This experimental 3D amplification factor provides means to take into account instabilities effect on large scale turbulent models. • The first quantification of the asymmetry factor in Hele-Shaw cells is presented. • Asymmetric cases may occur when the channel gap exceeds the cut-off wavelength. • The curvature between the plates results in an increase of the 2D flame surface by 25%–50% • For the first time, we have estimated the 3D wrinkling factor for unstable flames. • Amplification factor measurements show values greater than one for lean H2 mixture. [ABSTRACT FROM AUTHOR]